A study of the effect of local scour on the flow field near the spur dike

The flow field near a spur dike such as down flow and horseshoe vortex system(HVS)are susceptible to the topographic changes in the local scouring process,resulting in variation of the sediment transport with time.In this study,large eddy simulations with fixed-bed at different scouring stages were conducted to investigate the changes in flow field.The results imply that the bed deformation leads to an increase in flow rate per unit area,which represent the capability of sediment transportation by water,in the scour hole.Moreover,the intensity of turbulent kinetic energy and bimodal motion near the sand bed induced by the HVS were also varied.However,the peak moments between the two sediment transport mechanisms were different.Hence,understanding the complex feedback mechanism between topography and flow field is essential for the local scour problem.

Simulation of two-phase debris flow scouring bridge pier

Debris flows are typical two-phase flows, which commonly accompany erosion in mountainous areas, and may destroy bridge engineering by scouring. In this study, a physically-based two-phase model is applied for the simulation of debris flow scouring of bridge pier. In this model, the shear stress of debris flow on an erodible bed is considered to be a function of the solid shear stress, fluid shear stress, and volume fraction; accordingly, the erosion is incorporated into the two-phase model. Using a highaccuracy computational scheme based on the finite volume method, the model is employed for simulating a dynamic debris flow over an erodible bed. The numerical results are consistent with the experimental data, and verify the feasibility of the two-phase model. Moreover, a simple numerical test is performed to exhibit the fundamental behaviour of debris flow scouring of bridge pier, which shows that the degree of erosion on each side of the pier is higher compared to other areas. The scouring depth is influenced by the variations of solid volume fraction and velocity of debris flow and pier width.

基于Flow-3D的桩林坝不同布置形式对坝基的冲刷侵蚀

桩林坝是一种以桩为基础的竖向格子坝,既有良好的受力条件,又相对节约材料,同时拥有“拦粗排细”的功效,现有桩林坝的研究,多在冲击荷载、拦截固源和调控泥石流性能等方面构建不同布置形式的桩林坝模型。以杨家沟为例,通过野外现场踏勘和数值仿真计算,基于Flow-3D软件开展数值模拟研究,建立不同布置形式(单排桩、双排桩、桩间距、排间距)的桩林坝模型,探讨不同影响因素下桩林坝坝基冲刷侵蚀规律。结果表明:桩林坝的整体设计中桩林不宜过密,桩间距和排间距以2.5 m为宜;单排桩的稳定性较弱,在泥石流冲击下极易发生剪切破坏,且对上游的“固源”作用远不如多排桩,但单排桩的冲刷深度及侵蚀规模均小于多排桩。研究成果对桩林坝的布置形式及冲刷机制提供数据支撑,并为震后泥石流防治工程设计提供参考。

Scour of the Seabed Under A Pipeline in Oscillating Flow

The scour of the seabed under a pipeline is studied experimentally in this paper. Tests are carried out in a U-shaped oscillatory water tunnel with a box imbedded in the bottom of the test section. By use of the standard sand, clay and plastic grain as the seabed material, the influence of the bed material on the scour is studied. The relationship between the critical initial gap-to-diameter ratio above which no scour occurs and the parameters of the oscillating flow is obtained. The self-burial phenomenon. which occurs for the pipeline not fixed to two sidewalls of the test section, is not observed for the Bred pipeline. The effect of the pipe on sand wave formation is discussed. The maximum equilibrium scour depths For different initial gap-to-diameter ratios, different Kc numbers and different bed sands are also given in this paper.

Effects of spillway types on debris flow trajectory and scour behind a sabo dam

Debris flows are one of the common natural hazards in mountainous areas.They often cause devastating damage to the lives and property of local people.The sabo dam construction along a debris flow valley is considered to be a useful method for hazard mitigation.Previous work has concentrated on the different types of sabo dams such as close-type sabo dam,open-type sabo dam.However,little attention has been paid to the spillway structure of sabo dam.In the paper,a new type of spillway structure with lateral contraction was proposed.Debris flow patterns under four different spillway structures were investigated.The projection theory was employed to predict trajectory of debris flow out from the spillway and to estimate the incident angle and terminal velocity before it plunged into the scour hole behind the sabo dam.The results indicated that the estimated data were in good agreement with the experimental ones.The discrepancy between the estimated and experimental values of main parameters remained below 21.82%(relative error).Additionally,the effects of debris flow scales under different spillway structures were considered to study the scour law.Although the debris flow pattern and scour law behind the sabo dam under different operating conditions was analyzed in this paper,further study on the scour mechanism andthe maximum scour depth estimation based on scour theory is still required in the future.

Local Scour and the Laws of Scour Pit's Shape Downstream of Debris Flow Sabo Dam

The erosion shape and the law of development of debris flow sabo dam downstream is a weak part in the study on debris flow erosion. The shape and development of scour pit have an important effect on the stability and safety of debris flow sabo dam, which determines the foundational depth of the dam and the design of protective measures downstream. Study on the scouring law of sabo dam downstream can evaluate the erosion range and reasonably arrange auxiliary protective engineering. Therefore, a series of flume experiments are carried out including different debris flow characteristics （density is varying from 1.5 t/m3 to 2.1 t/m~） and different gully longitudinal slopes. The result shows that the scour pit appears as an oval shape in a plane and deep in the middle while superficial at the ends in the longitudinal section, the position of the maximum depth point moves towards downstream with an increase of flume slope angle. The maximum depth of scour pit is mainly affected by the longitudinal slope of gully, density of debris flow, and the characteristics of gully composition （particle size and the viscosity of soil）. The result also indicates that the viscosity of soil will weaken the erosion extent. The interior slopes of scour pit are different between the upstream and the downstream, and the downstream slope is smaller than the upper one. For the viscous and non-viscous sands with the same distribution of gradation, the interior slope of non- viscous sand is smaller than the viscous sand.According to tbe regression analysis on the experimental data, the quantitative relationship between the interior slope of scour pit, slope of repose under water and the longitudinal slope of gully is established and it can be used to calculate the interior slope of scour pit. The results can provide the basis for the parameter design of the debris flow control engineering foundation.

Calculation of the ultimate depth of a scour pit after debris flow through drainage canal ribs

Drainage canals are engineering structures widely used for debris flow mitigation.When passing through a drainage canal,debris flow usually scours the gully bed at the back of the rib sill of the drainage canal,which leads to failure of the rib sill.Therefore,the scour depth at the back of the rib sill is an important design problem and it is related to the economic benefits of engineering and service years.To explore the law of the depth of the scour pit after debris flow through drainage canal ribs,we first proposed a formula for the calculation of the maximum scour depth at the back of a rib sill based on energy conservation.We then conducted a series of simulation experiments to test the proposed formula.The experimental results show that the scour depth,trench slope and the distance between ribs all increase with a decrease in debris flow density.We then compared the results of experiments and formula calculations.Through the testing analysis,we found that the calculation results of the conductedformula correspond with the experimental results better.Finally,taking Qipan Gully as an example,we designed the ultimate depth of a drainage canal for debris flow using the calculation formula.

Experimental investigation of turbulent flows around high-rise structure foundations and implications on scour

Many studies have been undertaken to predict local scour around offshore high-rise structure foundations(HRSFs),which have been used in constructing the Donghai Wind Farm in China.However,there have been few works on the turbulent flow that drives the scour process.In this study,the characteristics of the turbulent flow fields around an HRSF were investigated using the particle image velocimetry technique.The mean flow,vorticity,and turbulence intensity were analyzed in detail.The relationship between the flow feature and scour development around an HRSF was elaborated.The results showed that the flow velocity increased to its maximum value near the third row of the pile group.The shear layer and wake vortices could not be fully developed downstream of the last row of the piles at small Reynolds numbers.The strong flow and turbulent fluctuation near the third piles explained the existence of a longtail scour pattern starting from the HRSF shoulders and a trapezoidal deposition region directly downstream of HRSF.This laboratory experiment gains insight into the mechanism of the turbulent flow around HRSFs and provides a rare dataset for numerical model verifications.

Investigation of Scouring Mechanism Around A Circular Pier

In the present study, the flow field around a circular pier is investigated with experimental measurements and numerical simulations. The transient flow patterns during erosion are studied in detail. The results show that the traditional equations of particle motion are not perfect for the calculation of the sand motion under this complex flow situation. The scouring agents, such as turbulent intensity, the fluctuating pressure and the vertical pressure gradient, having many effects on the sand motion with the increasing scouring depth, need to be considered in modifying the traditional model.

Bridge Pressure Flow Scour at Clear Water Threshold Condition

Bridge pressure flow scour at clear water threshold condition is studied theoretically and experimentally.The flume experiments reveal that the measured scour profiles under a bridge are more or less 2-dimensional;all the measured scour profiles can be described by two similarity equations,where the horizontal distance is scaled by the deck width while the local scour by the maximum scour depth;the maximum scour position is located just under the bridge about 15% deck width from the downstream deck edge;the scour begins at about one deck width upstream the bridge while the deposition occurs at about 2.5 deck widths downstream the bridge;and the maximum scour depth decreases with increas-ing sediment size,but increases with deck inundation.The theoretical analysis shows that:bridge scour can be divided into three cases,i.e.downstream unsubmerged,partially submerged,and totally submerged.For downstream unsubmerged flows,the maximum bridge scour depth is an open-channel problem where the conventional methods in terms of critical velocity or bed shear stress can be applied;for partially and totally submerged flows,the equilibrium maximum scour depth can be described by a scour and an inundation similarity number,which has been confirmed by experiments with two decks and two sediment sizes.For application,a design and field evaluation procedure with examples is presented,including the maximum scour depth and scour profile.

基于Rhino和Flow-3D耦合的串列四桥墩局部冲刷数值模拟

针对复杂地形下串列四排桥墩局部冲刷问题,以洺河京港澳高速桥6号桥组为研究对象,以不同泥沙级配及中值粒径表征非均匀沙质河床及来沙条件,考虑泥沙水下休止角等影响因素,通过建模软件Rhino耦合Flow-3D开展三维数值模拟研究,得到串列四排桥墩周围流场结构及冲坑演变过程。将模拟得到的流速值和冲深值与物理试验结果对比,相对误差均在10%以内,表明构建的数学模型能够较好地实现复杂地形下非均质河床的四排桥墩局部冲刷数值模拟。模拟方法可为类似三维曲面数值模拟研究提供参考,对于识别桥墩周围水流结构和冲刷特性、指导桥梁规划设计具有重要的参考价值。

Experimental Study on Engineering Behavior of Solidified Soil for Scour Repair and Protection

A new scour countermeasure using solidified slurry for offshore foundation has been proposed recently.Fluidized solidified slurry is pumped to seabed area around foundation for scour protection or pumped into the developed scour holes for scour repair as the fluidized material solidifies gradually.In the pumping operation and solidification,the engineering behaviors of solidified slurry require to be considered synthetically for the reliable application in scour repair and protection of ocean engineering such as the pumpability related flow value,flow diffusion behavior related rheological property,anti-scour performance related retention rate in solidification and bearing capacity related strength property after solidification.In this study,a series of laboratory tests are conducted to investigate the effects of mix proportion(initial water content and binder content)on the flow value,rheological properties,density,retention rate of solidified slurry and unconfined compressive strength(UCS).The results reveal that the flow value increases with the water content and decreases with the binder amount.All the solidified slurry exhibits Bingham plastic behavior when the shear rate is larger than 5 s^(-1).The Bingham model has been employed to fit the rheology test results,and empirical formulas for obtaining the density,yield stress and viscosity are established,providing scientific support for the numerical assessment of flow and diffusion of solidified slurry.Retention rate of solidified slurry decreases with the water flow velocity and flow value,which means the pumpability of solidified slurry is contrary to anti-scour performance.The unconfined compressive strength after solidification reduces as the water content increases and binder content decreases.A design and application procedure of solidified soil for scour repair and protection is also proposed for engineering reference.

Assessment of Turbulence Models on Bridge-Pier Scour Using Flow-3D

The main purpose of this paper is to analyze the influence of different turbulence flow models on scouring pit of bridge-pier. Flow-3D software is applied in line with the purpose. The key motivation for this study is to contribute to the Flow-3D software by means of some modification and adjustment in the sediment scour model and shallow water model. An assessment of turbulence model adopted with the parameters of the Melville experiment to estimate the maximum scour-depth was performed. In the simulation results, the alternate eddy formation and shedding were repeated while the Karman vortex street formed behind the pier for the large eddy simulation LES turbulence model is more realistic in the flow phenomenon. The results of the scour development of large eddy simulation (LES) turbulence model were found to be more satisfied than the Renormalized group (RNG) turbulence model and close to the prior experiment results. The simulated scour results were significantly different with the observed data collected from previous literature in the reason of some unsuitability of meshing method in Flow-3D software.

基于Flow-3D的水平射流冲刷泥沙数值模拟

为了模拟分析闸口水平射流对下游泥沙冲刷的全过程,基于水工模型试验,采用Flow-3D软件中的泥沙冲刷模型结合RNG模型和FAVOR方法,进行了水平射流冲刷固定底板下游泥沙的模拟试验。根据原水工模型条件,结合模型试验结果,对冲刷过程中的水面线、流速分布、冲坑深度和堆丘高度进行了对比分析。结果显示,数值模拟与原试验结果中水面线和流速分布吻合度很高,冲刷过程中的冲坑深度与堆丘的高度也很接近。分析了冲刷过程中的泥沙运动,模拟了不同粒径泥沙的冲刷过程并将结果进行对比,发现达到冲刷平衡阶段时冲坑深度与堆丘高度随泥沙粒径的增大而减小,该结论符合理论分析,证明该软件中泥沙模型可用于物理试验中的有关变量分析。

基于Flow-3D的东安工程桩坝冲刷数值模拟研究

透水桩坝因其独特的缓流落淤效果而在黄河流域得到广泛应用,其作为不抢险建筑物设计,但在多年的运行过程中,黄河下游复杂的冲淤情况使透水桩坝出现了不同程度的磨损、位移,东安工程甚至还发生了桩排倒塌。为厘清透水桩坝出险原因,利用Flow-3D软件对东安工程分别进行入流角度为45°、60°、90°的冲刷数值模拟,结合工程现场采用多波束测深系统测量所得数据,实现了透水桩坝冲刷仿真模拟的精准建模。结果表明:60°、90°入流角度的冲刷深度与实测断面冲刷深度拟合较好,最大误差仅为9.4%,模拟结果比较理想;Flow-3D软件对透水桩坝冲刷模拟有较好的适用性。研究成果可为透水桩坝的设计和后期安全运行提供依据。

细沙短时射流冲刷的FLUENT与FLOW-3D数值模拟方法比较

针对疏浚工程中的短时射流冲刷问题,探究何种数值模拟方法更为适合研究,分别采用FLUENT与FLOW-3D进行模拟,对两种模拟方法的理论模型进行了对比,并将数值模拟得到的冲坑形态、深度、截面积、体积与相应实验结果进行了比较。通过比较发现:FLUENT与FLOW-3D在网格划分、泥沙相模拟、水沙相互作用方面存在区别;FLUENT与FLOW-3D所得冲坑形态均与实验较为吻合,但由于FLUENT只对泥沙的起动和沉降进行设置,不能控制泥沙的悬浮和推移,而FLOW-3D中将泥沙看做个体,并可对泥沙的起动、沉降、悬浮和推移进行准确描述,所以相对于FLUENT,FLOW-3D得到的冲坑深度、截面积、体积等冲坑尺寸和发展过程,与实验更为接近;研究成果可以为细沙短时射流冲刷提供参考。

基于FLOW-3D的单桩海上风电基础防冲刷数值模拟研究

随着近年海上风电的快速发展,海上风电基础的防冲刷问题越来越引起重视。为深入分析海上风电单桩基础的冲刷过程及防冲刷措施的效果,利用FLOW-3D三维模拟软件对有无防护下的单桩基础进行数值模拟并和模型实验对比。结果表明:FLOW-3D可以较好地反映各防护措施下的冲刷过程,与模型实验结果吻合良好;抛石和软体排在一定条件下可以有效防护海床冲刷。研究可为海上风电基础的冲刷计算和防护设计提供参考。

基于Flow-3D垂直射流冲刷数值仿真技术研究

通航浅水区域的水下生产系统可采用沉箱进行防护,沉箱上方回填一定的土体,检修时需将回填土利用非接触式挖沟机冲开。建立三维射流冲刷数学模型是研究防护装置回填土冲开的一项基础性工作。文章利用Flow-3D建立了三维射流水动力与泥沙冲刷模型,并采用清水射流试验数据和射流冲刷试验数据对模型进行验证。数值模拟结果表明,所建立的模型可以较好地反映喷管下射流的水动力条件、泥沙运动与冲刷坑的发展变化规律。随后,将模型应用于检修工况下防护沉箱上方回填土的射流冲开研究,研究表明目前挖沟机的射流强度足以满足回填土厚1 m时的检修工况。

Flow Separation and Vortex Dynamics in Waves Propagating over A Submerged Quartercircular Breakwater

The interactions of cnoidal waves with a submerged quartercircular breakwater are investigated by a ReynoldsAveraged Navier–Stokes(RANS) flow solver with a Volume of Fluid(VOF) surface capturing scheme(RANSVOF) model. The vertical variation of the instantaneous velocity indicates that flow separation occurs at the boundary layer near the breakwater. The temporal evolution of the velocity and vorticity fields demonstrates vortex generation and shedding around the submerged quartercircular breakwater due to the flow separation. An empirical relationship between the vortex intensity and a few hydrodynamic parameters is proposed based on parametric analysis. In addition, the instantaneous and time-averaged vorticity fields reveal a pair of vortices of opposite signs at the breakwater which are expected to have significant effect on sediment entrainment, suspension, and transportation,therefore, scour on the leeside of the breakwater.

基于Flow-3D的圆柱形桥墩局部冲刷大涡模拟

为预测圆柱形桥墩周围的局部冲刷坑形态和最大冲坑深度,基于Flow-3D软件的水动力学模块和泥沙输运模块对桥墩附近局部冲刷进行了三维数值模拟。以Melville冲刷试验为原型,采用LES大涡模拟方法,模拟了桥墩附近湍流流场。以床面瞬时切应力作为泥沙起动、输移条件,采用Van Rijn输沙率公式计算床面冲淤。采用FAVOR技术追踪河床形态变化,得到了桥墩附近局部冲刷形态。经实测资料验证,计算结果与模型实测的冲坑形态及最大冲坑深度基本吻合。